(1) Field of the Invention
The present invention relates to a void-free polyurethane elastomer for the flat-free pneumatic tire. More particularly, the invention relates to a urethane-urea type tire filling material having an excellent Lupke rebound and an appropriate hardness.
(2) Description of the Prior Art
In many areas of severe service, "downtime" due to flat tires is a real problem. To maximize equipment operating efficiency and minimize "downtime" of pneumatic tire machines, a flat-free tire prepared by filling an elastomer is better than a pneumatic tire. Accordingly, use of a flat-free tire is desirable in such severe operational areas as aircraft support equipment, farm equipment, industrial vehicles such as forklift and tractors, lumbering operations, mining machinery and vehicles, military vehicles, scrap metal yard vehicles and computer controlled vehicles.
When a urethane type polymer is used as a filling material, a liquid prepolymer is injected in a cavity of a rimmed tire and the prepolymer is reacted in the tire casing to an appropriate hardness. Accordingly, processing of a tire by use of a urethane polymer as the tire filling material is advantageously very easy.
A urethane elastomer that has heretofore been used is prepared from a polyester or polyether having a terminal hydroxyl group (hereinafter referred to as "--OH group") and a diisocyanate, that is, polyurethane having a polyether or polyester as the skeleton. For example, "TYRFIL" (trademark of the product of Synair Co.) is said to be a reaction product of a mixture of polyoxypropylene glycol and polyoxypropylene triol with tolylene diisocyanate (hereinafter referred to as "TDI").
Such conventional urethane type elastomers have, however, the following defects.
(1) A tire filling material is required to have a high Lupke rebound and an appropriate hardness. In conventional urethane type elastomers, if the Lupke rebound is increased, the hardness becomes too great. On the other hand, if the hardness is reduced, the Lupke rebound is also reduced.
(2) If the urethane elastomer repeatedly receives a stress under a load when used as a filling material in a tire, heat is produced and the urethane elastomer becomes incapable of resisting the stress, resulting in deterioration, melting, and loss of the predetermined elasticity and spring rate.
To moderate these defects, a method in which a layer of a halogenated butyl rubber is formed between the urethane elastomer and rubber has been proposed, based on the concept that deterioration of the urethane elastomer is due to transfer of an amine type antioxidant contained in the rubber of a tire to the urethane side. However, this solution is not entirely satisfactory.
There has also been proposed a method in which an isocyanurate ring ordinarily regarded as having a good heat resistance is formed simultaneously with formation and curing of a urethane elastomer. However, it is questionable whether an isocyanurate ring will be actually formed under mild conditions adopted for formation of a urethane elastomer, and even if an isocyanurate ring is actually formed, since the concentration of the isocyanurate ring is low, no substantial improvement of the heat resistance can be expected. Furthermore, if the amount formed of the isocyanurate ring is increased so as to improve the heat resistance, the hardness, Lupke rebound and elasticity will depart from the desired ranges.
Three patents directed to urethane type elastomer filled tires in severe service are cited as prior art of interest, U.S. Pat. No. 3,866,651; U.S. Pat. No. 3,854,516 and U.S. Pat. No. 3,866,652.